1. Field of the Invention
The invention relates to a method for monitoring an alternating-voltage discharge on a double electrode, especially between two targets for cathodic sputtering. The method is used to avoid a change-over of the discharge from the state of the anomalous glow discharge to an arc discharge.
2. Discussion of Background Information
In this connection, the double electrode is basically an arrangement for an alternating-voltage discharge between two individual electrodes. The individual electrodes are placed alternatingly on cathodic and anodic potential. The method-related use of the alternating-voltage discharge can serve different purposes. On the one hand, the parameters and the electrode materials can be selected in such a way that essentially the plasma being formed with the discharge is used technically. On the other hand, the parameters and the electrode materials used can be selected in such a way that these materials are always sputtered on the cathodic side by the discharge. The latter method, cathodic sputtering or simply sputtering, is used in this method to deposit the electrode materials as a thin layer on a substrate. In practice, the electrodes are designated in cathodic sputtering as targets, corresponding to the target material.
The frequency, i.e., the phases of switching the electrodes between anodic and cathodic potential, is normally between 10 and 80 kHz in practice.
The progression of the current or voltage curves is symmetrically sinusoidal for the most part, but can also deviate therefrom. The impedance can also be asymmetrical, for example, if different types of target materials are to be deposited in parallel at different speeds (sputtering rates).
The invention can be used for simple cathodic sputtering devices as well as for the more effective magnetic field-supported magnetrons.
The use of double electrodes for cathodic sputtering is preferred in facilities in which very high sputtering rates are required so that, on the other hand, high deposition rates can be achieved when coating selected substrates. For example, these types of devices are operated as double magnetrons in a reactive atmosphere to deposit insulating layers.
Basically, two methods are known from the prior art for generating an alternating-voltage discharge. It is preferred for two direct-current sources with opposed potential to be alternatingly switched on the two electrodes of the double electrode. The method is also designated as Pulse Magnetron Sputter Process (PMS) (Schiller et al. xe2x80x9cPulsed Magnetron Sputter Technologyxe2x80x9d, Surface Coating Technology, 61, pages 331-337, 1993). On the other hand, the alternating-voltage discharge can be generated with a medium frequency generator, which basically generates sinusoidal current and/or voltage curves.
The invention is used for monitoring the alternating-voltage discharge, especially the monitoring of a discharge in which an alternating-current generator is used as an energy source. However, the method can also be used for monitoring an alternating-voltage discharge with two direct-current sources.
The problem in operating an alternating-voltage discharge on a double electrode with an alternating-current generator is that, in the case of the often very high electrical power used in practice, the discharge must continuously lie in the area of the anomalous glow discharge. A change-over of the discharge to an arc discharge may not take place. The anomalous glow discharge is the prerequisite for the discharge taking place over the entire effective cathode surface. This is the prerequisite for a uniform, error-free, and large-surface coating of the substrate or the substrate arrangement.
While, with an alternating-voltage discharge with two direct-current sources, the cathodic and anodic potentials are made available separately and always start from a zero potential, an alternating-voltage discharge with an alternating-current generator can drift to one electrode. As a result, the glow discharge can assume an asymmetry between the targets in which one target assumes a higher cathodic potential without the character of the sinusoidal current-voltage curve changing considerably. As a result, the danger of forming an arc discharge on the respective target increases.
The causes of a change-over of the discharge from an anomalous glow discharge to an arc discharge are diverse and cannot be completely excluded in practical operation. Thus, the causes of malfunctions could lie, for example, in the formation of flakes of sputtered material that flake off device parts and/or cause local overheating on the electrode surfaces. Malfunctions occur due to the short-term growth of insulating layers on the target. Additional malfunctions can be a function of the device or switching.
The arc discharge, also called vacuum arc discharge, is characterized in that it essentially has a lower impedance than the anomalous glow discharge, i.e., with the same power, the arc drop voltage is considerably lower and the discharge current is correspondingly greater. The arc discharge is connected with a punctiform vaporization of electrode material, which should be avoided in the process technology of cathodic sputtering under discussion. In this connection, the danger of the formation of an arc discharge exists in an increased degree with reactive process management.
Monitoring the power supply for individual magnetrons using measuring techniques is known from the prior art, in which the total current and/or voltage is measured and, when absolute values are exceeded, particularly with respect to the discharge current, the energy supply to the device is briefly interrupted.
Rettich and Wiedemuth (xe2x80x9cHigh power generators for medium frequency sputtering applicationsxe2x80x9d, Internet http://huetinger.com/mf_paper2.doc, /Jun. 11, 1997/) describe the requirements for a power supply for a double magnetron with a medium frequency generator. In this connection, the discharge is monitored by means of measuring the total discharge current and the discharge voltage. A possible change-over of the anomalous glow discharge to an arc discharge is reacted to in two different ways, which are a function of the type of arc. Accordingly, an individual short-term arc (single arc) is tolerable, which, though it causes an increase in current, extinguishes again after a waiting period that can be set in advance. In such a case, the generator operates as an energy source without interruption. If an excessive discharge current is measured via an arc discharge (hard arc), which exceeds the limit for the discharge current and the waiting period, the generator reacts within milliseconds by reducing the output power for a short time. As a result, the arc is extinguished. Afterwards, the normal method conditions are reset. If an arc discharge forms again, the generator is shut down and an error message is issued for the operator.
DE 43 26 100 A1 discloses a method and a device for coating substrates with a device for detecting and suppressing undesired electric arcs. In this connection, recording the measured values in a specific part of the half-wave is emphasized as a special feature that increases the reliability of detecting an arc.
It cannot be determined with the total current measurement of discharge current whether the discharge is occurring symmetrically or drifting asymmetrically to a target without the total current changing considerably. In such a case, the targets are sputtered at different speeds and it can easily come to a change-over of the glow discharge to an arc discharge, for example, due to the overheating of one of the targets.
In general, according to prior art, the discharge voltage, the discharge current or both in combination can be detected and, in the case of an impermissible deviation, the supply of energy to the discharge device can be briefly interrupted.
In this connection, it is problematic that, in the case of high power with frequencies of 10 to 80 kHz, various misinterpretations can occur for the anode/cathode switch-over and the interruption of the energy supply to the discharge devices frequently occurs unnecessarily in a disruptive manner. The recording of the discharge voltages and/or discharge currents that change with high frequency is influenced in practice in many cases by negligible, summary fluctuations that are not the result of the beginning of an arc discharge.
However, unnecessary, frequent shutting down of the double electrode due to misinterpretations of the recorded method parameters leads to undesirable process instabilities, which, with complicated coating tasks, can lead to the formation of defective, non-homogenous layers.
If the tolerance ranges of the monitored measured values are increased as a consequence of the described state of affairs, the danger exists, on the other hand, that, despite monitoring, a change-over from the anomalous glow discharge to an arc discharge does indeed occur, the supply of energy is not interrupted and, consequently, the technological task is disturbed in an impermissible manner.
As a result, the invention is directed to a method for monitoring a double electrode, with which a change-over or development into a change-over of an alternating-voltage discharge from the state of an anomalous glow discharge to an arc discharge can be detected with a high degree of certainty.
The present invention provides a process similar in general to the process discussed above which also includes that the discharge current and/or the discharge voltage of each of the two half-waves is/are measured within an alternating-voltage discharge period and the difference between the respective measured values of the second half-wave and the corresponding measured values of the first half-wave are compared to specific tolerance values. When the tolerance values are exceeded, the power supply is reduced so that the discharge is briefly suppressed.
Advantageous further developments of the invention are characterized in the sub-claims and are described in greater detail in the following along with a description of a preferred design of the invention in the exemplary embodiment.
The core of the invention is that the discharge current and/or the discharge voltage of each of the two half-waves within a period of the alternating-voltage discharge is recorded as a measured value. In the case of a deviation of the measured value of the subsequent half-wave from the measured value of the preceding half-wave from a method-specific tolerance value, the discharge is briefly interrupted. As compared to prior art, it is no longer necessary for a static or dynamically carried comparison value to be prescribed for the still-tolerable discharge current and/or discharge voltage. As a result, the reliability of the detection of the beginning of an arc discharge is increased considerably.
In further embodiments of the invention the ratios of discharge current and discharge voltage of the two electrodes can be recorded and compared to corresponding tolerance values.
In the case of certain method-specific ratios, it can also be advantageous to compare a summary or average measured value over several half-waves with a corresponding measured value of preceding half-waves. The measured value formation can take place, for example, via two or five half-waves. With a larger number of half-waves, it can already come to the undesired formation of an arc. In detail, these types of variations depend considerably upon concrete technological conditions and circumstances.
The essential advantage of invention is that not only the total current can be monitored, but the discharge current on every electrode for each half-wave. Of particular significance in this connection is the reliable detection of asymmetrical discharge currents.
Various smaller deviations from absolute tolerance values, which individually do not lead to a change-over of the glow discharge to an arc discharge, can be eliminated as non-essential and the process flow can be continued undisturbed without shutting down. In many cases, despite relatively large deviations from the prescribed process parameters, there is actually no concrete danger of a change-over to an arc discharge.
However, as soon as, due to circumstances, the discharge current of a half-wave increases above a tolerance value as compared to the preceding half-wave, it must be assumed that this is not merely a regulation-related shift in the process parameters or small process-induced malfunctions and the discharge is interrupted so that no arc discharge can form.
The instant invention is directed to a process for monitoring an alternating-voltage discharge between the electrodes of a double electrode. The process includes measuring values of at least one of a discharge current and a discharge voltage for each half-wave within an alternating-voltage discharge period, determining a difference between the measured values of a second half-wave and the measured values of the first half-wave, and comparing the determined differences to specific tolerance values. When the specific tolerance values are exceeded by the determined differences, a power supply is reduced, whereby the discharge is at least briefly suppressed.
According to a feature of the present invention, the alternative-voltage discharge can be monitored between two targets for cathodic sputtering.
In accordance with another feature of the invention, the first half-wave value may be a positive current.
Moreover, the determined difference may be a difference between one-half of a magnitude of the at least one of the discharge current and the discharge voltage between a peak of the first half-wave and a peak of the second half-wave and the measured value of the second half-wave of the at least one of the discharge current and the discharge voltage.
Further, the determined difference can be,a difference between one-half of a total discharge current between peak values of the first and second half-waves and the measured value of the second half-wave of the discharge current.
The determined difference can also be a difference between one-half of a total discharge voltage between peak values of the first and second half-waves and the measured value of the second half-wave of the discharge voltage.
According to still another feature of the present invention, the determined difference may be determined from ratio values of different measured values of the measured half-waves to specific ratio tolerance values.
In accordance with a further feature of the invention, the determined difference between one or more periods and the determined difference between one or more preceding periods can be compared to specific tolerance values.
Further, an average of determined differences of one or more periods may be compared to the specific tolerance values.
At least one of a total discharge current and a total discharge voltage between peaks of the first and the second half-waves can remain substantially constant.
The present invention is also directed to an apparatus. The apparatus includes a double magnetron including first and second targets arranged to form a double electrode, and a power supply coupled to supply power to the first and second targets. The power supply includes a measurement unit for measuring values of at least one of discharge current and discharge voltage, such that at least one of a discharge current and a discharge voltage for each half-wave within an alternating-voltage discharge period is measured. A device for determining a difference between the measured values of a second half-wave and the measured values of the first half-wave is provided, as well as a device for comparing the determined differences to specific tolerance values, and a device for at least briefly suppressing the discharge when the specific tolerance values are exceeded by the determined differences.
According to a feature of the instant invention, a switching element may be coupled between the power supply and the first and second targets. The switching element may include a measuring element for recording a duration of the half-waves.
In accordance with another feature of the present invention, the first and second targets can include titanium.
According to still another feature of the invention, the first and second targets can have an overall electrode area of 12,000 cm2.
Moreover, a vacuum chamber can be provided, within which the double magnetron may be located. The vacuum chamber can include inlets for forming a reactive oxygen atmosphere within the vacuum chamber, and a working pressure of 3xc3x9710xe2x88x921 Pa can exerted within the vacuum chamber.
In accordance with yet another feature of the present invention, the device for at least briefly suppressing the discharge can include a device for reducing the power supplied to the first and second targets. by the power supply.
The invention is explained in greater detail in the following based upon an exemplary embodiment.